lemonie is totally correct. Vinegar is a result of naturally occuring bacteria turning alcohol (ethanol) into acetic acid. Most websites tell you to add Mother of vinegar to start the process, but you don't actually need to, this just speeds it up. Mother of vinegar will actually form in your vinegar, it's a blob of thick bacteria, best to scoop it off before using your vinegar, you can even use it to speed up your next batch. Exposure to air is the key to the process. I made some alcohol using household yeast, sugar and water, and then once it stopped fermenting I exposed it to air for about 2-3 months. It made reasonably strong vinegar with a slightly yeasty odour. Ironically I didn't mean to make vinegar, but then later decided to, checked my homebrew and found I had already made some. And I got a mouthful of mother of vinegar when I first tasted it.